1. Field of the Invention
The present invention relates to a light-emitting device that has a light-emitting element.
2. Description of the Related Art
Commercialization of organic EL displays is accelerating. The required luminance of displays is becoming higher year by year for outdoor use. It is known that the luminance of an organic EL element increases in proportion to electric current, and light emission at high luminance can be achieved.
However, a large current flow accelerates deterioration of organic EL elements. Thus, if high luminance can be achieved with a small amount of current, light-emitting elements can have longer lifetimes. In this regard, a tandem element in which a plurality of light-emitting units is stacked has been proposed as a light-emitting element capable of providing high luminance with a small amount of current (see Patent Document 1, for example).
Note that in this specification, a light-emitting unit refers to a layer or a stacked body which includes one region where electrons and holes injected from both ends recombine.
A tandem element including two light-emitting units, for example, can emit light equivalent to that of a light-emitting element including one light-emitting unit by current with half the density of the light-emitting element including one light-emitting unit. In addition, a structure in which n light-emitting units are stacked between electrodes, for example, can provide n times the luminance of one light-emitting unit without increasing current density.
One problem of a light-emitting panel in which tandem elements are provided adjacently is occurrence of a crosstalk phenomenon. The crosstalk phenomenon refers to a phenomenon in which, when a light-emitting layer and a layer having higher conductivity than that of the light-emitting layer are provided in a plurality of sub-pixels as common layers, current leaks to an adjacent sub-pixel through the layer with higher conductivity. For example, when a tandem element has a structure in which a first light-emitting unit including a first light-emitting layer and a second light-emitting unit including a second light-emitting layer are stacked and an intermediate layer having higher conductivity than the conductivity of the first light-emitting layer and that of the second light-emitting layer is provided between the first and second light-emitting units, current leaks to an adjacent tandem element through the intermediate layer. Note that a problem of a crosstalk phenomenon similarly occurs in a single element in which a carrier-injection layer (a hole-injection layer or an electron-injection layer) is used as a common layer.
A tandem element includes stacked layers with a highly conductive intermediate layer therebetween, thereby including a layer with high conductivity and a layer with low conductivity in the structure. In addition, in the tandem element, a mixed layer of an organic compound and a metal oxide or a highly conductive carrier-injection layer of a conductive high molecular compound or the like is often used in order to decrease driving voltage. Furthermore, electrical resistance between an anode and a cathode is higher in the tandem element than in a single element; thus, current is easily transmitted to an adjacent pixel through the highly conductive layer.
FIG. 8A is a schematic diagram for describing the crosstalk phenomenon caused by a highly conductive intermediate layer 86. In the cross-sectional view of FIG. 8A, three stripes of tandem elements that emit white light are arranged in a light-emitting panel (white-light-emitting panel) and only a second tandem element is driven.
The light-emitting panel includes first to third tandem elements which are adjacent to one another. The first tandem element is provided between an upper electrode 81 and a first lower electrode 82. The second tandem element is provided between the upper electrode 81 and a second lower electrode 83. The third tandem element is provided between the upper electrode 81 and a third lower electrode 84.
In each of the first to third tandem elements, a first light-emitting unit 85, the intermediate layer 86, and a second light-emitting unit 87 are sequentially stacked. For example, when the first light-emitting unit 85 includes a light-emitting layer that emits blue light and the second light-emitting unit 87 includes a light-emitting layer that emits green light and a light-emitting layer that emits red light, each tandem element can provide white light emission.
In the case of using a light-transmitting upper electrode, a counter glass substrate 88 can be arranged over the upper electrode and reflective electrodes can be used as the lower electrodes. The counter glass substrate 88 is provided with a blue color filter, a red color filter, and a green color filter (not illustrated). The red color filter, the blue color filter, and the green color filter overlap with the first lower electrode 82, the second lower electrode 83, and the third lower electrode 84, respectively.
When only the blue line (the second tandem element) is driven in the above-described light-emitting panel by application of voltage between the second lower electrode 83 and the upper electrode 81, current might leak to the adjacent first or third tandem element through the highly conductive intermediate layer 86, causing the red line (the first tandem element) or the green line (the third tandem element) to emit light and a crosstalk phenomenon to occur.
FIG. 8B is a schematic diagram for describing the crosstalk phenomenon caused by a highly conductive carrier-injection layer (a hole-injection or electron-injection layer) 89. In FIG. 8B, only a blue line (a second tandem element) is driven in a light-emitting panel (white-light-emitting panel).
In each of first to third tandem elements, the first light-emitting unit 85 including the highly conductive carrier-injection layer 89, the intermediate layer 86, and the second light-emitting unit 87 are sequentially stacked. As an example of the carrier-injection layer 89, a highly conductive layer containing a mixed material of an organic compound and a metal oxide, a conductive high molecular compound, or the like can be given.